As we enter the fourth decade of the HIV epidemic, significant advances have been made in the understanding of HIV pathogenesis and in the development of potent and safe antiviral drugs. More than 30 antiviral drugs have been registered and the impact of combination antiretroviral therapy (ART) on both morbidity and mortality has been remarkable. However, despite the long-term suppression of HIV replication achieved in patients with optimal adherence to ART, HIV invariably rebounds after interruption of therapy. Furthermore, successful therapy does not induce or allow restoration/development of virus-specific immune responses capable of controlling HIV replication in the absence of ART. Thus, life-long ART is needed to control HIV replication and associated disease in the large majority of HIV infected subjects.
A population of long-lived central memory CD4 T-cells latently infected with HIV has been identified in the blood as an important component of the HIV cell reservoir and as the primary cause of HIV persistence. The life-span of this latent cell reservoir is estimated to be approximately 70 years in the presence of full HIV suppression with ART. However, recent studies have demonstrated that two populations of CD4 T-cells resident in lymph nodes serve as the primary CD4 T-cell compartment for HIV infection, replication and production. These two CD4 T-cell populations are defined by the expression of PD-1 and CXCR5 and include the PD-1+CXCR5+, i.e. T follicular helper cells (Tfh) and PD-1+CXCR5− CD4 T-cell populations.
A number of mechanisms responsible for the establishment and maintenance of the HIV latent cell reservoir(s) have been proposed. One of the mechanisms is the persistent of minimal virus replication under ART which may replenish the HIV cell reservoir. Therefore, ART is unable to induce full suppression of HIV replication and the “natural” HIV-1 specific immune response under ART is also unable to totally suppress and eliminate ongoing residual virus replication. The failures of ART and of the HIV-specific immune response provide the rationale for investigating alternative interventions to target also the persistent HIV cell reservoir.
A number of immunological interventions have been investigated in the past and currently being further developed with the goal to achieve HIV functional cure, wherein viral replication is suppressed without sustained antiviral therapy (9). Therapeutic vaccine strategies have been the primary intervention strategy investigated but the results have shown modest efficacy in experimental animal models and patients with the exception of a CMV-based vector HIV vaccine (50% efficacy in the NHP model; 10). Recent studies have generated interesting results on the possibility of using anti-envelope broad neutralizing antibodies (bNabs) as therapeutic agents in HIV infection (11,12). Furthermore, antagonist PD-1 Abs have been shown to restore T-cell functions in HIV infected patients and the possibility to use these Abs as a therapeutic strategy to augment the potency of HIV-specific T-cell responses has been proposed (13,14).
It is well established that infiltrating tumor-specific CD8 T-cells are dysfunctional with regard their ability to proliferate and to mediate cytotoxic activity. The large majority of infiltrating tumor-specific CD8 T-cells are in a so-called exhaustion functional state. The primary mechanism responsible for the exhaustion of infiltrating tumor-specific CD8 T-cells is the increased expression of a number of regulatory receptors and particularly PD-1 regulatory receptor. The observation that the blockade of the PD-1/PDL-1/2 (PD-1 ligands) is associated with the recovery of CD8 T-cells from exhaustion has provided the rationale for developing intervention strategies targeting the PD-1 molecule expressed by exhausted CD8 T-cells. Recent studies have shown very promising results with the use of PD-1 antibodies with antagonist activity in patients with advanced cancer-associated disease. Studies have show substantial rates of response, ranging from 18 to 40%, in patients with advanced melanoma, non-small cell lung carcinoma and renal carcinoma. Anti-PD-1 antibodies in these studies have been used either alone or in combination with an anti-CTL-A4 antibody. After these initial studies, the current studies are being performed in patients with a variety of tumors including also hematological tumors.
There is a need in the art for additional reagents for targeting PD-1 and methods for using the same. This disclosure addresses those needs by providing reagents and methods that may be used to target PD-1 and cells and/or tissues expressing the same.